VisibilityATC Chapter 5

Aim

To learn about Visibility and how it affects us as pilots.

Objectives

1.Define Slant visibility and horizontal visibility

2.Define Runway Visual Range3. Explain factors affecting visibility4. Describe fog and mist formation

BackgroundVisibility is extremely important for us as visual pilots to know whether a flight can be carried out safely or legally.If the visibility decreases below a specified amount en-route, we would not be able to see where we are going, therefore would not be able to fly.If the visibility at our arrival aerodrome is below minimums, an alternate would be required.

1. Slant and horizontal vis.Slant Visibility and Horizontal Visibility

If an aircraft is approaching an airfield in low visibility above a layer of stratus or fog, the pilot may be able to see the airfield from directly above the airfieldAs soon as the aircraft is on approach to land, the airfield may be hard to see because the observer is looking through a thicker layer of weather, this is known as slant visibility

Horizontal visibility is the visibility given at the same level to the station.Visibility reported at an aerodrome, or given in a forecast is horizontal visibilityPilot’s should exercise extreme caution when operating on the border of VMC as they may not be able to land as they see slant visibility

2. Runway Visual RangeRunway Visual Range is defined as the maximum distance along a runway

that can be seen from a position corresponding to the average pilots eye level at touchdownRVR is broadcast at certain aerodromes with precision approaches ie the ILS.RVR is given in metres along a runway from touchdown zone.

3. Factors affecting VisibilityThere are numerous factors both natural and man made which affect how far

we can see.These factors include:

• Fog and mist• Precipitation• Spray from the sea• Smoke haze• Dust and sand storms

3. Factors affecting Visibility

Precipitation can occur in many different ways and in many different forms• It can occur as rain, drizzle, hail or snow

The degree of the reduction in visibility will depend on the amount and size of the water dropletsIf flying in rain, the visibility can reduce to as little as 50m in very heavy rain and up to over 10km in drizzle.Hail is virtually invisible so the risks of flying in hail is not the reduction in visibility

Precipitation

Visibility in snow can vary from below 1000m in heavy snow.It is very important to study the weather available to us and make decisions whether flight can be undertaken through showers.

3. Factors affecting Visibility

Spray from the sea is not possible from normal flight over water as you would have to be very low for it to affect you.Spray from the sea reduces visibility for those seaside airports and is not actually water that reduces visibility, it is the salt that is deposited on windscreens after it is evaporated from the water.

Spray from the sea

3. Factors affecting Visibility

The content of smoke is usually carbon and sulphurThese together are dark in colour and reduce visible lightIf there was an inversion, soot can build up to very high concentrations and reduce visibility sometimes below VMC!

Smoke Haze

3. Factors affecting Visibility

A dust storm is defined as a condition in which the visibility is reduced to below 1000m by raised dust,The requirements for formation of a duststorm are:

• A long period of dry weather• Surface wind exceeding 15kts• Unstable air (steep lapse rate)

• The steeper the lapse rate, the greater the depth in which the dust is raised.

Dust and Sand Storms

3. Factors affecting Visibility

A temperature inversion can act as a blanket in which vertical convection is stopped.That is air that is warmed starts to rise but meets air equal in temperature and stops.Particles that are trapped in the inversion will stay there as they will not be able to rise any further.These particles could be anything from smoke, haze or pollution.

Inversions

These particles can act as condensation nuclei and if the relative humidity is high enough, fog can form.

• If the condensation nuclei is from smoke, and the relative humidity is high enough, smog will form (combination of smoke and fog)• Smog can be found mostly in

industrial areas.

4. FogFog is the most frequent cause of low visibility at airports, as such it is one of the most important hazards to aviation.In general, fog is low level cloud and can severely reduce the visibility to less than 1000mThe relative humidity in fog is 100%Mist is much the same but the visibility is not less than 1000m and the relative humidity is just below 100%.The different types of fog which we will discuss are:

• Radiation fog• Advection fog• Frontal fog• Upslope fog• Steam fog

4. Fog

Radiation fog is caused when the air is cooled to below the dewpoint due to heat loss from radiation.Conditions favourable for the formation of radiation fog are:

• A cloudless night (allowing the land to lose heat by radiation to the atmosphere)

• Cold land surface• A high relative humidity (only a little bit of cooling is required for air to

reach saturation)• Light winds (promotes mixing of air)• The presence of condensation nuclei

Radiation Fog

4. Fog

Radiation fog is dissipated when the sun heats the earth up causing the air above it to also warmIf the fog is very thick it can act as a blanket insulating the ground. The ground will not be able to warm, therefore the air above it will not warm. In this scenario the fog can last throughout the dayFog can dissipate if the wind strength is increased therefore creating enough turbulence to drag warmer air down into the fog layer.

Radiation Fog

4. Fog

Advection is to do with the horizontal movement of air.When a moist air mass moves horizontally and comes into contact with a colder surface the air is cooled from belowIf the cooling is significant enough to cool the air to below the dewpoint, fog will form.Fog formed in this way is termed advection fogMoist air may flow from a relatively warm sea to a cold air mass to form advection fog

Advection Fog

4. Fog

Frontal fog occurs at the boundary of two air masses rather than within a single air mass as previously discussed with radiation and advection fog.It occurs as cloud reaching or descending to the surface at the frontal edge, or forms in the precipitation when the air becomes saturated

Frontal Fog

4. Fog

Steam fog forms when a cold moist air mass flows over a much warmer water bodyThe evaporation of water from the warm water surface causes the air above the surface to become saturated.Because the surface is warmer, convection currents give the steamy appearance.

Steam Fog

4. Fog

When moist is forced up a slope or mountain it is forced to cool adiabatically.If the moist air reaches the dewpoint, fog will form.If the wind stops the fog will dissipate.

Upslope Fog

Questions?